Optical scanning systems or optical scanning devices using an optical deflector including a movable element which performs sinusoidal vibration have conventionally been proposed. The optical scanning systems using an optical deflector which performs sinusoidal vibration have advantageous features, in comparison with optical scanning systems using a rotary polygonal mirror such as a polygon mirror, that: the optical deflector can be made very small in size; the power consumption is slow; and the optical deflector made of silicon monocrystal and produced by the semiconductor process has theoretically no metal fatigue and the durability is very good.
In optical deflectors using a resonance phenomenon, by exciting two or more natural oscillation modes in the torsional oscillation direction, optical scan other than the optical scanning of sinusoidal wave can be provided (see U.S. Pat. No. 7,271,943). FIG. 12 is a top plan view for explaining such optical deflector. A movable member (first movable element) 1001 of planar shape is supported by two torsion springs 1011a and 1011b at its top and bottom as viewed in the drawing, and it has a permanent magnet 1041. A movable member (second movable element) 1002 of a frame-like shape supports the torsion springs 1011a and 1011b at the inside thereof. It is supported by two torsion springs 1012a and 1012b at its top and bottom as viewed in the drawing. A supporting frame 1021 of frame-like shape supports the torsion springs 1012a and 1012b at the inside thereof. The supporting frame 1021 is bonded to a plate member 1000.
The movable members 1001 and 1002 and the torsion springs 1011 and 1012 have two natural oscillation modes, and the frequency ratio of them is in the relationship of approximately 1:2. By exciting these two modes at the same time, the optical deflector is driven by sawtooth wave oscillation, such that the optical scanning can be done with small drift of angular speed.
However, in the optical deflector of FIG. 12, no specific attention has been paid to the disturbance (which may be called “jitter”) of the oscillatory motion of the movable element caused by any turbulence of a airflow around the movable member when it is oscillated at a large deflection angle or by a vibration entered from the outside.
On the other hand, as an optical deflector which can stabilize the displacement angle of the movable element and the oscillatory motion thereof, there is an optical deflector such as shown in FIG. 13 which is an exploded view (see Japanese Laid-Open Patent Application No. 2003-057586). In FIG. 13, the optical deflector 1 comprises an oscillator 5, a base table 2 and a cover 20. The oscillator 5 includes a reflecting mirror (movable element) 8, torsion springs 9 and 10 coupled to the mirror, and a fixed frame member 7 to which the torsion springs 9 and 10 are coupled. The base table 2 includes supporting members 3 and 4, concaved portions 2a and 2b formed at the top surface between the supporting members 3 and 4, and electrodes 11 and 12 provided on the concaved portions 2a and 2b to oscillate the torsion spring 10. The cover 20 which covers the oscillator 5 is made of a material through which a light beam (not shown) to be deflect can be transmitted.
In such an optical deflector, when the reflecting mirror 8 oscillates and torsionally displaces, the stress produced at the connection point between the torsion springs 9 and 10 and the fixed frame member 7 can dispersed. Also, the stress can be dispersed to the torsion spring 10 as well without stress concentration only at the torsion spring 9. Thus, the torsion springs 9 and 10 need not be made too thick or too long. Thus, with a compact design, a comparatively large displacement angle (deflection angle) is provided while maintaining the resonance frequency of the reflecting mirror 8. Furthermore, by sealing the cover 20 and by thereafter filling it with a reduced pressure gas or inactive gas, any turbulence of the airflow can be reduced. In this way, the oscillatory motion of the oscillator can be stabilized.
In an electrophotographic apparatus such as a laser beam printer, an image is formed by scanning a photosensitive member surface with a laser beam. If an optical deflector such as shown in FIG. 12 is used in such apparatus and the scan is made with a large displacement angle, no particular measures have been taken to reduce the jitter of the oscillatory motion of the movable element due to a turbulence of the airflow or the like to thereby stabilize the oscillatory motion of the movable element.
On the other hand, in the case of the optical deflector shown in FIG. 13, turbulence of the airflow may be reduced by the reduced-pressure gas charging or inactive gas charging, and the displacement angle and the oscillatory motion may be stabilized thereby. However, the packaging for the reduced-pressure gas charging or inactive gas charging will lead to an increased manufacturing cost. Furthermore, since the cover is made of a material through which the light beam to be scanningly deflected by the reflection mirror surface can be transmitted, the quantity of the light beam inevitably drops to some extent.